T lymphocytes require dynamic movement for all of their critical functions. Motility, synapse formation and signaling are intricately linked through he actions of the cellular cytoskeleton. Data from our lab and others indicates that the T cell cortex is tightly regulated from within, using myosin motors and the associated septin cytoskeleton that control cortical integrity and membrane tension, all of which function via intimate association with a continuously remodeling actin cytoskeleton. This control has profound implications for the process of scanning organs for antigens, for the process of interacting with antigen-presenting cells, and for the process of interacting with targets. It is clear that there remains a dearth of understanding about which individual system controls T cell membrane biology and specifically how these: 1. Control cell-intrinsic scanning behavior such that organs are completely surveyed. 2. Control cortical and membrane tension so that membrane-membrane interactions (synapses) are optimized. 3. Drive the large-scale aggregation of proteins into domains that encourage signaling or lead to its cessation. This project will study these fundamental issues. We have assembled an unrivalled panel of genetic knockouts in the myosin/septin pathway and have revealed critical roles actin depolymerization as a co-factor in synapses and likely cell motility. This project will delve into how the T cell works in its native habitat and will reveal nvel mechanisms that regulate immunity and tolerance.